Remotely positioned slitter system



April 1965 H. R. PATTERSON, JR 3,176,566

REMOTELY POSITIONED SLITTER SYSTEM Filed June 2, 1961 5 Sheets-Sheet 1 INVEN TOR. Henry E. Pafzersm J:

()RNE YS April 1965 H. R. PATTERSON, JR 3,176,556

REMOTELY POSITIONED SLITTER SYSTEM Filed June 2, 1961 3 Sheets-Sheet 2 R. Henry 1?? Pal/ersaxz Jr:

3 Sheets-Sheet 3 April 6, 1965 Filed June 2, 1961 MMQTP lllllllllllll INMNAW VIII l q] INVENTOR.

l/epry R Palm/15m J:

BY a M a; Law ATTi RNEYS United States Patent 3,176,566 REMQTETJY PGSTTTGNED LHTER SYSTEM Henry R. Patterson, 31:, West Chester, Pa", assignor to lleloit Eastern Corporation, Downington, 1%., a corporation of Delaware Filed lune 2, 1961, Ser. No. 114,446 8 Claims. (Cl. 33-348) This invention relates to a slitter system wherein a web of paper or the like fed from a supply roll is separated into two or more narrower webs which may then be wound on separate rolls. With the system of this invention, slitter units can be positioned far more readily and much more accurately than is possible with systems of the prior art.

In prior systems, the adjustment of the width of the web involves a cumbersome manual adjustment of the position of a slitter unit, which may be supported on rails or the like extending transversely with respect to the web. The adjustment has been particularly difiicult in that each unit comprises upper and lower blades or bands which must be positioned with a high degree of accuracy relative to one another, to obtain proper shearing action. Such adjustment has usually involved the cooperation of two or three men, With time-consuming trial and error methods, and with the machine being idle during the adjustment. In addition, the prior art methods have resulted in marring of guide rails to an extent such that setting the units into the proper position has been increasingly difiicult.

According to this invention, severing means are moved transversely relative to a web by power operated means so controlled as to position the severing means with a high de ree of accuracy. The power operated means preferably comprises an electric motor on a carrier for he severing means, coupled to a pinion which meshes with a rack on a transverse guide rail. A sensing device, also mounted on the carrier, develops a signal corresponding to the position of the severing means. This signal is compared with a command signal to develop a control signal which is amplified and used to control drive of the pinion. The command signal may preferably be developed by manual rotation of a control knob on a control panel.

With this arrangement, the severing means can be readily positioned, and a high degree of accuracy is obtained. The positioning can be effected while the web is in motion and it is not necessary to use the trial and error methods of the prior art. In addition, there is no marring of the guide rail.

An important feature of the invention is that upper andlow-er slitter units, having cooperating shearing blades, can be simultaneously positioned. Furthermore, the pressure between the blades of the units can be controlled with a high degree or" accuracy to obtain proper shearing action.

Another important feature relates to the provision of lock ng means for locking the severing means in position, after the desired position is reached.

This invention contemplates other and more specific obiects, features and advantages which will become more fully apparent from the following detailed description taken in conjunction with the accompanying drawings which illustrate a preferred embodiment and in which:

FIGURE 1 is a diagrammatic side elevational view of a paper slitting and winding machine adapted to use the control system of this invention;

FIGURE 2 is an enlarged view of the slitter units of the machine of FIGURE 1 and shows diagrammatically the connection of the units through electrical cables to control units mounted on a panel;

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FIGURE 3 is another elevational View of the upper and lower slitter units shown in FIGURE 2, showing the form or" the rack mounting on guide rails and also showing how two or more pairs of slitter units may be em ployed; and

FEGURE 4 is an electrical circuit diagram.

As shown on the drawings: 7

The remotely controlled slitter system of this invention, generally designated by reference numeral 16, comprises a slitter blade 11 which is supported from above a paper web 12 and which shearingly engages a slitter band 13 supported from below the paper web 12. As shown in FIGURE 1, the paper web 12 may travel from a supply spool under an adjustable lead-in roll 14, thence over a guard board 15 and thence between the blade 11 and the cooperating band 12 to be severed by shearing action, The separate portions of the severed web then travel over a guard board 16, and thence downwardly about a roll 17 to be wrapped about cores 1% removably mounted on uprights 19. Before reaching the roll 17, the separate severed portions of the web maybe spread apart by an adjustable spreader bar 2! The moving sheet or web 12 is slit or severed into a plurality of desired widths and in accordance with this invention a plurality of slitting blades 11 and cooperating slitting bands 13 may be positioned in transversely spaced relation along the frame of the apparatus. The frame may take various forms but may preferably comprise a non-rotating crossbeam 21 of generally circular cross section attached to a stand 22. A support structure 23 is provided to support the guard boards 15 and 16 and the slitter band 13 is supported therefrom in a manner to be described.

Referring now to FIGURES 2 and 3, the slitter blade 11 is supported from a slide member 24 which in turn is supported on a rail 25 which is welded or otherwise secured to the crossbeam 21 with spacer blocks 26 being disposed between the rail 25 and the crossbeam 21. The slitter band 13 is supported on a slide member 27 which is supported on a rail 28 of the lower support structure 22 Rack and pinion means are provided for adjusting the positions of the slides 24 and 27 along the rails 25 and 28. In particular, a pinion 29, journalled within the slide member 24, engages a rack 39 on the rail 25. A pinion 31 is journalled in a depending portion 274 of the slide 27 and meshes With a rack 32 which is supported in fixed relation to the rail 28.

To drive the pinions 29 and 31, they are respectively coupled through clutch, gearbox and sensing units 33 and 34 to electric motors 35 and 36. The units 33 and 34 are, by themselves, conventional. Each comprises a pair of electrically operated clutches which may be selectively energized to couple the motor through reduction gearing to the pinion and thus move the associated slide in one direction or the other. Each of the units 33 and 34 may alsoinclude a sensing device to develop an electric signal which indicates the position of the slide along the associated rail. An additional sensing device in the form of a command synchro 37 may be supported on the top slide 24 and connected to the pinion 29 to develop a signal which is used in controlling the position of the bottom slide in a manner as described below.

The motors 35 and 36 are connected through power lines 33 and 39 to a source of power, and the units 33 and 34 are connected through control lines 40 and 41 to command synchro and amplifier units 42 and 43, supported on a panel 44. Knobs d5 and 46 and switches 47 and 48 are disposed on the fronts of the units 42 and 43.

In the general operation of the system, switch 47 is turned to an ON position and the motors 35 and 36 3 are energized. To shift the position of the blade 11, the knob 45 may be rotated in one direction or the other to rotate a selsyn generator within the unit 42. Signals are then applied through the line 40 to the clutch, gearbox and sensing unit 33 to develop in a control transformer of the unit 33 an error signal corresponding to the difference between the position of the knob 45 and the position of the blade 11. This error signal is applied back through the control line 40 to an amplifier system of the unit 42 to develop a signal which is applied again through the line 40 to one or the other of a pair of clutches in the unit 33. The pinion 29 is then rotated in the proper direction to move the blade to a position corresponding to the set position of the knob 45.

The control of the lower unit is dependent upon the position of switch 48. In one position of the switch, the selsyn generator within unit 42 is directly connected to a control transformer in the unit 34, so that the lower unit is directly controlled by the knob 45 of the upper panel unit 42. In another position of switch 48 the position of the unit is controlled by knob 46 in the same way that the position of the upper unit is controlled by the knob 45. In a third position of the switch 48, the upper unit controls the position of the lower unit. In this operation, the sensing unit 37, in the form of a selsyn generator, is connected through line 49 and through contacts of the switch 48 and through line 41 to a control transformer of the clutch, gearbox and sensing unit 34.

When both the upper and lower units are in proper positions, the switch 47 is then thrown to an OFF position and, according to an important feature of the invention, signals are applied to clutches of the units 33 and 34 to lock both units in position, while the motors 3S and 36 are deenergized.

Referring now to FIGURE 4, the selsyn generator of the unit 42 is generally designated by reference numeral 50. It comprises a rotor coil 51 connected through conductors 52 and 53 to a suitable source of alternating current and three stator coils 54, 55 and 56 disposed in 120 spaced relation. The stator coils 54-56 of the generator 50 are connected through conductors in the line 40 to corresponding stator coils 57, 58 and 59 of a control transformer 60 in the clutch, gearbox and sensing unit 33.

The control transformer 60 has a rotor 61 mechanically connected to the pinion 29 and electrically connected through conductors in the control line 40 to input terminals of an amplifier 62 in the unit 42. The amplifier 62 has a pair of terminals 63 and 64 connected to the AC. supply lines 52 and 53 and has three output terminals. Terminal 66 is a common terminal connected to terminals of both a forward clutch 68 and a reverse clutch 69 within the clutch, gearbox and sensing unit 33. Terminal 65 is connected to the other terminal of the forward clutch 68 while terminal 67 is connected to the other terminal of the reverse clutch 69.

The selsyn generator 50 and control transformer 60 are illustrated with the rotors 51 and 61 in a relative balanced condition. Inthis condition, a maximum voltage is induced in coil 54 aligned with the rotor coil 51 while equal smaller voltages are induced in the coils 55 and 56. In the control transformer 60, no voltage is induced in the rotor coil 61 because it lies at right angles to the coil 57 and the induced voltages from the coils 58 and 59 are equal and opposite. However, if the rotor coil 51 is rotated through its mechanical connection with the knob 45, a voltage will be induced in the coil 61, the magnitude of the voltage varying in accordance with the sine of the angle of shift of the coil 51. This voltage is applied to the input of the amplifier 62 and, depending upon the phase of the voltage, an output signal is developed either at 4 terminal or at terminal 67 to actuate either the forward clutch 68 or the reverse clutch 69. The pinion 29 is then driven in a direction to move the rotor coil 61 through an angle equal to the-angle of movement of the rotor coil 51. It may be noted that the amplifier should include a phase-sensitive detector, connected to terminals 63 and 64, which are connected to the supply lines 52 and 53.

Unit 43 comprises a selsyn generator 70 having a rotor coil 71 connected to the supply lines 52 and 53 and havin three stator coils 72, 73 and 74. In a lower position of the switch 48, coils 72, 73 and 74 are connected through contacts 48a, 48b and 480 of the switch 48 to stator coils 75, 76 and 77 of a control transformer 78 in the unit 34. The control transformer 78 has a rotor coil 79 which is connected to the inputs of an amplifier 80 having a pair of terminals 81 and 82 connected to the supply lines 52 and 53 and having three output terminals 83, 84 and 85. Terminal 83 is connected to a forward clutch 86 of the the unit 34, terminal 84 is connected to the other terminal of the clutch 86 and to one terminal of a reverse clutch 87, and terminal is connected to the other terminal of the reverse clutch 87. This system, of course, operates in the same manner as the system above described including selsyn generator 50, control transformer 60, amplifier 62 and clutches 68, 69, the position of the lower unit being controlled by the position of the rotor 71 connected to the knob 46.

In the upper position of the switch 48 the stator coils '7 5, 76 and 77 of the transformer 78 are connected through the switch contacts 48a, 48b and 48s to stator coils 88, 89 and 90 of the selsyn generator 37. Generator 37 has a rotor coil 91 mechanically coupled to the pinion 29 and electrically coupled to the supply lines 52, 53. It will be appreciated that with the switch 48 in the position as illustrated, the generator 37 performs the same functions as the generator 78 in the other position of the switch. In this case, the position of the lower unit is controlled from the position of the upper. unit.

In the intermediate position of theswitch 48, as illustrated, the coils 75, 76 and 77 of the lower unit control transformer 78 are connected to the stator coils 54-56 of transformer 50 in parallel with the stator coils 57-59, so that both units are directly controlled by knob 45.

As above indicated, it is oftentimes desirable to use a'plurality of pairs of slitter units in conjunction with a single winding apparatus. For example, as shown in FIGURE 3, another pair of slitter units including an upper blade 11' and a lower band 13' may be supported on the rails 25 and 28. In such cases, it is desirable to automatically deenergize the drive units of the slitters when one is moved against another. Furthermore, it is desirable to deenergize the drive when the slitters reach a certain limit of travel in one direction or another. According to this invention, a pair of switches 92 and 93 are mounted on opposite sides of the upper unit to engage either a fixed stop or another upper slitter unit, the switches being opened upon such engagement. Switches 92 and 93 are respectively connected between amplifier terminals 65 and 67 and the forward and reverse clutches 68 and 69, to thus automatically deenergize such clutches. Similarly, a pair of switches 94 and 95 are connected between amplifier terminals 83 and 85 and the forward and reverse clutches 86 and 87 of the lower unit 34, switches 94 and 95 being physically positioned on opposite sides of the bottom unit. Thus switches 92 and 94 may engage switches 93' and 95' of the adjacent pair of slitter units illustrated in FIGURE 3, while switches 93 and 94 may engage fixed stops 93a and 94a in the illustrated system.

Means are provided for energizing all of the clutches upon deenergization of the drive motors, to lock the units in position. For this purpose, terminals of the 'clutches 68, 69, 86 and 87 are respectively connected through relay contacts 96, 97, 98 and 99 to a terminal 100 arranged for connection to a suitable current source the other terminal of which is connected to a terminal 101 which is connected to the common terminals of the clutches 68 and 69 and the common terminals of the clutches 86 and 87. The contacts 96-99 are normally closed but are opened upon energization of a relay coil connected in parallel with the motors 35 and 36. Accordingly, when the motors 35 and 3% are energized by closing the switch 4'7, to close contacts 47a and 47b thereof, the relay 162 is energized to open contacts 96-99 and permit positioning operation of the clutches. However, when switch contacts 47:: and 471) are opened, the relay 1632 is deenergized to thereafter allow the contacts 95-9 to close, thereby applying current to all of the clutches. With all clutches energized and with the motors deenergized, the units are locked in position because of high frictional resistance to rotation in the units 33 and 34 combined with the motors 35 and 36, there being a large speed reduction in the gearboxes of the units.

The system or" this invention is particularly advantageous in combination with the illustrated form of slitter structure, wherein the blade 11 is in the form of an essentially flat disc having opposed surfaces 163 and 164 connected by a tapered or conical peripheral surface 195 which defines a sharp cutting edge. The peripheral edge portion of the surface 1&3 engages a peripheral edge portion of the lower band 13, to obtain the shearing action. The disc 11 and the band 13 are preferably rotated and for this purpose, the band 13 is mounted on the shaft of a motor 1% which is supported on the slide 27. The disc 11 is then driven by frictional contact with the band 13, the disc 11 being carried between a hub member 167 and a nut N38 mounted on a shaft 199 which is carried by a member 116, depending from the upper slide 24. A thumb out 111 isprovided to adjust the vertical position of the disc 11, after which a locking nut 112 is operated.

With this type of slitting structure, an extremely en' icient action is obtained over long periods of time. It is, however, necessary that the proper pressure be obtained between the blade 11 and the band 13, and this pressure can be obtained with the precision positioning system of this invention, whereas it is difilcult to obtain by the manual adjustment systems of the prior art. In addition, the proper tension can be maintained while shifting the positions of the slitter units, because of the concurrent control thereof with the system as above described.

lt will be understood that modifications and variations may be efiected without departing from the spirit and scope of the novel concepts of'this invention.

I claim as my invention:

1. In a severing system including means for supporting a web of material for longitudinal movement, web-severing means including a pair of shearing means, a pair of carrier means supporting said shearing means from opposite sides of said web, a pair of guide means supporting said shearing means for movement transversely with respect to said web, a pair of sensing means for developing a pair of signals corresponding to the transverse positions of said pair of carrier means, command means for developing a command signal corresponding to the desired position of said carrier means, a pair of power means for moving said pair of carrier means, and means responsive to any difierence between each signal of said pair of signals and said command signal and arranged to respectively control said power means to drive said carrier means along said guide means and develop a balance between said signals.

2. In a severing system including means for supporting a web. of material for longitudinal movement, web-severing means including first and second shearing means, first and second carrier means supporting said first and second shearing means from opposite sides of said web, first and second guide means supporting said shearing means for .movement transversely with respect to said web, first and second sensing means for developing first and second signals corresponding to the transverse positions of said first and second carrier means, means for developing a first command signal corresponding to a desired position "of said carrier means, means for developing a second command signal corresponding to the transverse position of said first carrier means, first and second power means for moving said first and second carrier means, means responsive to any dilference between said first signal and said first command signal and arranged to control said first power means to drive said first carrier means along said first guide means and develop a balance between said first signal and said first command signal, and means responsive to any difference between said second signal and said second command signal and arranged to control said second power means to drive to said second carrier means along said second guide means and develop a balance between said second signal and said second command signal.

3. In a severing system including means for supporting a web of material for longitudinal movement, web-sew ering means, carrier means carrying said severing means, guide means supporting said carrier means for movement transversely with respect to said web, power means for moving said carrier means along said guide means, means for developing a first signal corresponding to the transverse position of said carrier means, command means for developing a signal corresponding to a desired position of said carrier means, means responsive to any difference between said signals and arranged to control said power means to drive said carrier means along said guide means and develop a balance between said signals, movemerit-limiting means on said guide means in the path of movement of said carrier means, and interlock means on said carrier means engageable with said movement-limiting means and arranged to disable said power means.

4. A web slitting mechanism for slitting a continuous flexible web travelling along a'path comprising, a support frame having means for guiding a longitudinally travelling web, means on the frame defining a slitting stationextending across the web including a plurality of rotary slitter bands including first and second bands rotatable about a laterally extending axis for supporting the travelling web each having an annular side cutting edge, a plurality of circular rotary slitting knives including first and second knives rotatable about a laterally extending axis and supported above the band for individually coacting with individual bands to longitudinally slit the web, first and second lower carrier members respectively supporting said first and second bands and being laterally shiftable of the path of web movement, first and second upper carrier members respectively supporting said first and second knives and being laterallyshiftable, upper and lower guide means respectively guiding said upper and lower carrier members and extending parallel transversely of the web path, first and second separate motors respectively connected to each of said firstand second upper carrier members for independently individually moving the knives along the guide means in either direction, first and second individual motors respectively connected to said lower carrier members for individually moving each of the lower carrier members along its guide means in either direction, and separate electrical control switch means positioned remote from said motors at a control station and having independently operable first and second upper motor circuits and first and second lower motor circuits connected to separately selectively operate individual of said motors so that either an individual band or knife may be positioned and the other band or knife aligned therewith independent of other knives and bands to form coacting slitting couples at desired spaced intervals whereby each band and its coacting knife may be brought together to obtain a required cutting pressure therebetween.

5. A web slitting mechanism for slitfing a continuous flexible web travelling along a path comprising, asupport frame having means for guiding a longitudinally travelling web, means on the frame defining a slitting station extending across the web including a plurality of rotary slitter bands including first and second bands 3,1 mass 3 rotatable about a laterally extending axis for supporting the travelling tweb each having an annular side cutting edge, a plurality of circular rotary slitting knives including first and second knives rotatable about a laterally extending axis and supported above the band for individudividually moving the knives along the guide means in either direction, first and second individual motors respectively connected to said lower carrier members for individually moving each of the lower carrier members along its guide means in either direction, upper and lower racks extending along said upper and lower guide means, individual gear trains connected to individual motors and driving pinions in mesh with the racks, said gear trains having sufficient frictional resistance to lock the knives and the bands in position against each other with the motors inoperative during cutting operation of the knives and bands, and separate electrical control switch means positioned remote from said motors at a control station and having independently operable first and second upper motor circuits and first and second lower motor circuits connected to separately selectively operate individual of said motors so that either an individual band or knife may be positioned and the other band or knife aligned therewith independent of other knives and bands to form coacting slitting couples at desired spaced intervals whereby each band and its coacting knife may be brought together to obtain a required cutting pressure therebetween.

6. A web slitting mechanism for slitting a continuous flexible web traveling along a path comprising, a support frame having means for guiding a longitudinally travelling web, means on the frame defining a slitting station extending across the web including a plurality of rotary slitter bands including first and second bands rotatable about a laterally extending axis for supporting the traveling web each having an annular side cutting edge, a plurality of circular rotary slitting knives including first and second knives rotatable about a laterally extending axis and supported above the band for individually coacting with individual bands to longitudinally slit the web, first and second lower carrier members respectively supporting said first and second bands and being laterally shiftable of the path of web movement, first and second upper carrier members respectively supporting said first and second knives and being laterally shiftable, upper and lower guide means respectively guiding said upper and lower carrier members and extending parallel transversely of the web path, first and second separate motors respectively connected to each of said first and second upper -carrier members for independently individually moving the knives along the guide means in either direction, first and second individual motors respectively connected to said lower carrier members for individually moving each of the lower carrier members along its guide means in either direction, driving means for coupling each. of said motors to the carriers to move them along the guide means, a pair of forward and reverse individually selectively operable clutch means coupling each of said motors to said driving means to move the knives and bands in opposite transverse directions, separate electrical control switch means positioned remote from said motors at a control station and having independently operable first and second upper motor circuits and first and second lower motor circuits connected to separately selectively operate individual of said motors so that either an individual band or knife may be positioned and the other band or knife aligned therewith independent of other knives and bands to form coacting slitting couples at desired spaced intervals whereby each band and its coacting knife may be brought together to obtain a required cutting pressure therebetween, and means for deenergizing the individual electric motors while energizing both of the clutch means to lock the driving means for individual knives and bands in a desired location.

7. A web slitting mechanism for slitting a continuous flexible web traveling along a path comprising, a support frame having means for guiding a longitudinally travelling web, means on the frame defining a slitting station extending across the web including a plurality of rotary slitter bands including first and second bands rotatable about a laterally extending axis for supporting the travelling web each having an annular side cutting edge, a plurality of circular rotary slitting knives including first and second knives rotatable about a laterally extending axis and supported above the band for individually coacting with individual bands to longitudinally slit the Web, first and second lower carrier members respectively supporting said first and second bands and being laterally shiftable of the path of web movement, first and second upper carrier members respectively supporting said first and second knives and being laterally shiftable, upper and lower guide means respectively guiding said upper and lower carrier members and extending parallel transversely of the web path, first and second separate motors respectively connected to each of said first and second upper carrier members for independently individually moving the knives along the guide means in either direction, first and second individual motors respectively connected to said lower carrier members for individually moving each of the lower carrier members along its guide means in either direction, driving means for coupling each of said motors to the carriers to move them along the guide means, selectively operable clutch means coupling each of said motors to its driving means to selectively move each of said bands or knives in opposite transverse directions, separate electrical control switch means positioned 'remote from said motors at a control location having independently operable first and second upper motor circuits and first and second lower motor circuits connected to separately selectively operate individual of said motors and clutch means so that either an individual band or knife may be positioned and the other band or knife aligned therewith independent of other knives and bands to form coacting slitting couples at desired spaced intervals whereby each band and its coacting knife may be brought together to obtain a required cutting pressure therebetween, movement limiting means in the path of movement of the individual knives and bands, and interlocked switch means in circuit with the control switch means which control said clutch means of the individual bands and knives to engage said movement limiting means edge, a plurality of circular rotary slitting knives including first and second knives rotatable about a laterally extending axis and supported above the band for individually coating with individual bands to longitudinally slit the web, first and second lower carrier members respectively supporting said first and second bands and being laterally shiftable of the path of web movement, first and second upper carrier members respectively supporting said first and second knives and being laterally shiftable,

upper and lower guide means respectively guiding said upper and lower carrier members and extending parallel transversely of the-web path, first and second separate motors respectively connected to each of said first and second upper carrier members for independently individually moving the knives along the guide means in either direction, first and second individual motors respectively connected to said lower carrier members for individually moving each of the lower carrier members along its guide means in either direction, separate electrical control switch means positioned remote from said motors at a control station and having independently operable first and second upper motor circuits and first and second lower motor circuits connected to separately selectively operate individual of said motors so that either an individual band or knife may be positioned and the other band or knife aligned therewith independent of other knives and bands to form coacting slitting couples at desired spaced intervals whereby each band and its coacting knife may be brought together to obtain a required cutting pressure therebetween, and means at said control station for l 0 indicating the instantaneous position of individual knives and bands.

References (Iited by the Examiner UNITED STATES PATENTS 1,333,588 3/20 Lewis 83-368 1,749,931 3/30 Wall 83-433 2,363,208 11/44 Sulzer 83-552 2,405,709 8/46 Olsson 192-143 2,747,666 5/56 Brooks 83-368 2,816,454 12/57 Hosea et a1. 192-84 2,872,980 2/59 Perrine 83-498 2,900,025 8/59 Lamb 83-399 3,036,483 5/62 Porter 83-498 FOREIGN PATENTS 1,019,525 11/57 Germany.

ANDREW R. JUHASZ, Primary Examiner. CARL W. TOMLIN, Examiner. 

1. IN A SEVERING SYSTEM INCLUDING MEANS FOR SUPPORTING A WEB OF MATERIAL FOR LONGITUDINAL MOVEMENT, WEB-SEVERING MEANS INCLUDING A PAIR OF SHEARING MEANS, A PAIR OF CARRIER MEANS SUPPORTING SAID SHEARING MEANS FROM OPPOSITE SIDES OF SAID WEB, A PAIR OF GUIDE MEANS SUPPORTING SAID SHEARING MEANS FOR MOVEMENT TRANSVERSELY WITH RESPECT TO SAID WEB, A PAIR OF SENSING MEANS FOR DEVELOPING A PAIR OF SIGNALS CORRESPONDING TO THE TRANSVERSE POSITIONS OF SAID PAIR OF CARRIER MEANS, COMMAND MEANS FOR DEVELOPING A COMMAND SIGNAL CORRESPONDING TO THE DESIRED POSITION OF SAID CARRIER MEANS, A PAIR OF POWER MEANS FOR MOVING SAID PAIR OF CARRIER MEANS, AND MEANS RESPONSIVE TO ANY DIFFERENCE BETWEEN EACH SIGNAL OF SAID PAIR OF SIGNALS AND SAID COMMAND SIGNAL AND ARRANGED TO RESPECTIVELY CONTROL SAID POWER MEANS TO DRIVE SAID CARRIER MEANS ALONG SAID GUIDE MEANS AND DEVELOP A BALANCE BETWEEN SAID SIGNALS. 